The muscle protein paramyosin occurs in a great variety of invertebrates and somewhat similar proteins may occur in vertebrates. The function of this protein is unknown. It occurs as the core of the thick filaments upon which myosin is deposited, therefore, it may have a structural role in organization of the thick filament. In addition, it appears to be involved in the interaction of myosin of the thick filament with actin of thin filaments. Hence, it may have a second role in regulation of the rate of muscle contraction-relaxation processes. However, our current knowledge is meager and even the binding of myosin to paramyosin has never been demonstrated in a direct fashion. The proposed research is designed to demonstrate that binding does occur, to determine the regions of both myosin and paramyosin essential for binding, and to determine the conditions that affect the tightness of binding. This information should greatly enhance our understanding of the true role of paramyosin.